The purpose of this study was to document the inaccuracy rate of diagnosis of cerebral palsy recorded on the South Australian Cerebral Palsy Register. A total of 402 children born in South Australia from 1993 to 2002 and notified to the Register as having cerebral palsy were identified through the Register database, and 21 children (5.2%) were later identified to have a noncerebral palsy diagnosis. Of these, 5 had either a metabolic or a neurodegenerative disorder and 2 had a syndromic disorder (1 Joubert syndrome and 1 Sotos syndrome); the remaining 14 children had one of the following final diagnoses: developmental delay, gross motor delay, perinatal myositis, spinal subdural and subarachnoid arteriovenous malformation, and Erb's palsy. In 16 of 21 children (76%), the diagnosis was changed at 5 years of age or older. Studies based on population registers may need to take into account the possibility of misclassification, estimated to be at least 5.2% in this study. A complete clinical assessment at the time of diagnosis followed by regular reassessment would enable the clinician to exclude children with alternative diagnoses, which has important implications for clinical management and research based on cerebral palsy registers.